Allow newer kernels to opt into explicit loop and chunk limit overrid…

…es (#2047)

Summary: Allow newer kernels to opt into explicit loop and chunk limit
overrides

Overwriting non default values of the bpf chunk and loop limit is
frustrating since it undoes configuration a user explicitly requested.
This also will allow for working around #2042, which causes kernels 6.10
and later to fail to start the socket tracer.

In addition to this, I tweaked the kernel upgrade logic slightly. The
previous logic would have upgraded a 5.0.x kernel to use the new loop
limit when the 1M instruction limit isn't available until 5.1.

Relevant Issues: Helps to work around #2042

Type of change: /kind bugfix

Test Plan: Verified the following on a 6.x kernel running
`stirling_wrapper`
- [x] Not supplying any arguments results in using an increased loop and
chunk limit
```
$ sudo ./bazel-bin/src/stirling/binaries/stirling_wrapper
I20241118 05:52:37.358364 3104175 socket_trace_connector.cc:474] Kernel version greater than V5.1 detected (6.8.12), raised loop limit to 882 and chunk limit to 84
```
- [x] Supplying the loop limit or chunk limit flag disables the
automatic increase
```
$ sudo ./bazel-bin/src/stirling/binaries/stirling_wrapper --stirling_bpf_loop_limit=41

I20241118 05:53:21.082810 3104197 source_connector.cc:35] Initializing source connector: socket_tracer
I20241118 05:53:21.082886 3104197 kernel_version.cc:82] Obtained Linux version string from `uname`: 6.8.0-1015-gcp
I20241118 05:53:21.082916 3104197 linux_headers.cc:395] Detected kernel release (uname -r): 6.8.0-1015-gcp
I20241118 05:53:21.082964 3104197 linux_headers.cc:206] Using Linux headers from: /lib/modules/6.8.0-1015-gcp/build.
I20241118 05:53:21.083058 3104197 bcc_wrapper.cc:166] Initializing BPF program ...
```

Changelog Message: Ensures that the `--stirling_bpf_loop_limit` and
`--stirling_bpf_chunk_limit` values are respected if explicitly provided
on the command line. For 5.1 and later kernels, cli provided values
would have been ignored

---------

Signed-off-by: Dom Del Nano <ddelnano@gmail.com>

src/stirling/source_connectors/socket_tracer/socket_trace_connector.cc

@@ -460,7 +460,14 @@ auto SocketTraceConnector::InitPerfBufferSpecs() {
 Status SocketTraceConnector::InitBPF() {
   // set BPF loop limit and chunk limit based on kernel version
   auto kernel = system::GetCachedKernelVersion();
-  if (kernel.version >= 5 || (kernel.version == 5 && kernel.major_rev >= 1)) {
+  auto loop_limit_is_default =
+      gflags::GetCommandLineFlagInfoOrDie("stirling_bpf_loop_limit").is_default;
+  auto chunk_limit_is_default =
+      gflags::GetCommandLineFlagInfoOrDie("stirling_bpf_chunk_limit").is_default;
+
+  // Do not automatically raise loop and chunk limits for non-default values.
+  if (loop_limit_is_default && chunk_limit_is_default &&
+      (kernel.version > 5 || (kernel.version == 5 && kernel.major_rev >= 1))) {
     // Kernels >= 5.1 have higher BPF instruction limits (1 million for verifier).
     // This enables a 21x increase to our loop and chunk limits
     FLAGS_stirling_bpf_loop_limit = 882;